1. Field of the Invention
The present invention relates to an emissive display device using emissive elements, such as electroluminescence elements, which employs thin film transistors for controlling such elements.
2. Description of the Related Art
In recent years, electroluminescence (referred to hereinafter as xe2x80x9cELxe2x80x9d) display devices comprising EL elements have gained attention as potential replacements for CRTs and LCDS. Research has been directed to the development of EL display devices using, for example, thin film transistors (referred to hereinafter as xe2x80x9cTFTxe2x80x9d) as switching elements to drive the EL elements.
FIG. 1 is a plan view showing one display pixel of an organic EL display device. FIG. 2 illustrates an equivalent circuit for a plurality of display pixels in an organic EL display device. FIG. 3A shows a cross-sectional view taken along line Axe2x80x94A of FIG. 1, while FIG. 3B shows a cross-sectional view taken along line Bxe2x80x94B of FIG. 1.
As shown in FIGS. 1, 2, 3A, and 3B, each display pixel is formed in a region surrounded by gate signal lines 151 and drain signal lines 152. A first TFT serving as a switching element is disposed near a intersection of those signal lines. The source 131s of the TFT simultaneously functions as a capacitor electrode 155 such that, together with the opposing storage capacitor electrode 154 described later, it forms a capacitor. The source 131s is connected to a gate electrode 142 of a second TFT 140 that drives the organic EL element. The source 141s of the second TFT 140 contacts with the anode 161 of the organic EL element. The drain 141d is connected to a power source line 153.
Near the TFT 130, a storage capacitor electrode 154 is disposed in parallel with a gate signal line 151. The storage capacitor electrode 154 is made of a material such as chromium. The storage capacitor electrode 154 contacts the capacitor electrode 155 via a gate insulating film 112 and together stores charges, forming a capacitor. The capacitor electrode 155 is connected to the source 131s of the first TFT 130. This storage capacitor is provided for retaining voltage applied to the gate 142 of the second TFT 140.
The first TFT 130, or the switching TFT, will now be explained.
As shown in FIGS. 1 and 3A, gate signal lines 151 made of refractory metal such as chromium (Cr) or molybdenum (Mo), which also serve as gate electrodes 132, are formed on an insulator substrate 110 made of quartz glass, non-alkali glass, or a similar material. Also disposed on the substrate 110 are drain signal lines 152 composed of aluminum (Al) and power source lines 153 also composed of Al and serving as the power source for the organic EL elements.
After forming gate signal lines 151 on the substrate 110, a gate insulating film 112 and an active layer 131 composed of poly-silicon (referred to hereinafter as xe2x80x9cp-Sixe2x80x9d) film are sequentially formed. The active layer 131 is of a so-called LDD (Lightly Doped Drain) structure. Specifically, low-concentration regions 131LD are formed on both sides of each gate 132. The source 131s and the drain 131d, which are high-concentration regions, are further disposed on the outboard sides of the low-concentration regions 131LD.
An interlayer insulating film 115 formed by sequential lamination of a SiO2 film, a SiN film, and a SiO2 film is provided on the entire surface over the gate insulating film 112, the active layer 131, and stopper insulating films 114. A contact hole formed in a position corresponding to the drain 141d is filled with metal such as Al, forming a drain electrode 116. Further, a planarizing insulating film 117 made of an organic resin or a similar material is formed over the entire surface for planarization.
The second TFT 140, or the TFT for driving the organic EL element, will next be described.
As shown in FIG. 3B, gate electrodes 142 composed of refractory metal such as Cr or Mo are formed on the insulator substrate 110 made of quartz glass, non-alkali glass, or a similar material. Further on top, a gate insulating film 112 and an active layer 141 composed of p-Si film are sequentially formed. The active layer 141 is provided with intrinsic or substantially intrinsic channels 141c formed above the gate electrodes 142, and the source 141s and drain 141d are formed on respective sides of these channels 141c by ion doping using p-type impurities, thereby constituting a p-type channel TFT.
An interlayer insulating film 115 formed by sequential lamination of a SiO2 film, a SiN film, and a SiO2 film is provided on the entire surface over the gate insulating film 112 and the active layer 141. A contact hole formed in a position corresponding to the drain 141d is filled with metal such as Al, forming a power source line 153 connected to a power source. Further, a planarizing insulating film 117 made of an organic resin or a similar material is formed over the entire surface for planarization. A contact hole is formed in the planarizing insulating film 117 in a position corresponding to the source 141s. A transparent electrode made of ITO (indium tin oxide) that contacts the source 141s through this contact hole, namely, the anode 161 of the organic EL element, is formed on the planarizing insulating film 117.
The organic EL element 160 is formed by laminating, in order, the anode 161 constituted by a transparent electrode made of ITO or similar material, an emissive element layer 166 which is composed with materials including an organic compound and comprises an emissive layer, and a cathode 167 made of a magnesium-indium alloy. The cathode 167 is disposed over the entire surface of the organic EL display element shown in FIG. 1, that is covering the entire sheet of the figure.
In an organic EL element, holes injected from the anode and electrons injected from the cathode recombine in the emissive layer. As a result, organic molecules constituting the emissive layer are excited, generating excitons. Through the process in which these excitons undergo radiation until deactivation, light is emitted from the emissive layer. This light radiates outward through the transparent anode via the transparent insulator substrate, resulting in light emission.
In this way, electric charge applied via the source 131s of the first TFT 130 is accumulated in the storage capacitor 170 and applied to the gate 142 of the second TFT 140. According to this voltage, the organic EL element emits light.
As shown in FIG. 2, each power source line connected to the power source for driving the organic EL elements is connected with a power source input terminal 180 disposed outside the display pixel region. The power source lines are arranged and connected with each vertical array of display pixels. With such an arrangement, at positions more distant from the power source input terminal 180 resistance of each power source line increases along with its length. The organic EL elements 160 located in display pixels distant from the power source input terminal 180 are therefore not adequately provided with necessary current, causing a disadvantage that the display in such area is dim.
The present invention was created in light of the above existing disadvantage. The purpose of the present invention is to provide an EL display device which prevents decrease in power source current due to resistance of power source lines, and adequately provides EL elements with current that should actually be supplied, accomplishing bright display.
To achieve the above purpose, the present invention provides an electroluminescence display device comprising a plurality of display pixels arranged in a matrix within a display pixel region, said display pixels having electroluminescence elements including an emissive layer between first and second electrodes, wherein, within said display pixel region, power source line for supplying power from a power source to said electroluminescence elements is disposed in a grid pattern.
According to another aspect of the present invention, said power source line includes main power source lines arranged in plural numbers within said display pixel region, and at least one bypass power source line extended to intersect and connect said main power lines within said display pixel region.
In a further aspect of the present invention, each of said display pixels further comprises a first thin film transistor having a gate electrode connected to a gate line, and a first electrode region connected to a data line; and a second thin film transistor having a gate electrode connected to a second electrode region of said first thin film transistor, a first electrode region connected to one of said main power source lines, and a second electrode region connected to said electroluminescence element.
In a still further aspect of the present invention, an emissive display device comprises a plurality of display pixels arranged in a matrix within a display pixel region, each of said display pixels having emissive elements including an emissive layer between first and second electrodes; wherein, within said display pixel region, power source line for supplying power from a power source to said emissive elements is disposed in a grid pattern.
According to another aspect of the present invention, in any one of the above-described devices, said power source line includes main power source lines arranged in plural numbers along the column direction of said matrix within said display pixel region, and at least one bypass power source line extended in the row direction of said matrix to intersect and connect a plurality of said main power lines.
According to a further aspect of the present invention, in any one of the above-described devices, each of said display pixels further comprises a first thin film transistor having a gate electrode connected to a gate line, and a first electrode region connected to a data line; and a second thin film transistor having a gate electrode connected to a second electrode region of said first thin film transistor, a first electrode region connected to one of said main power source lines, and a second electrode region connected to said electroluminescence element or said emissive element.
In another aspect of the present invention, there is provided an electroluminescence display device comprising a display pixel region having a plurality of display pixels arranged in a matrix. Each of said display pixels includes an electroluminescence element having an emissive layer between an anode and a cathode; a first thin film transistor having a gate electrode connected to a gate line, and a first electrode region connected to a data line; and a second thin film transistor having a gate electrode connected to a second electrode region of said first thin film transistor, a first electrode region connected to a power source line for supplying power from a power source to said electroluminescence element, and a second electrode region connected to said electroluminescence element. Said power source line is provided in plural numbers along the column direction of said matrix within said display pixel region, and those power source lines that are associated with the display pixels adjacently arranged along the row direction are connected to one another via a bypass power source line extending in the row direction of said matrix.
As described above, the power source line for supplying power (current or voltage) from the power source to emissive elements such as electroluminescence elements is arranged in a grid pattern. Alternatively, a plurality of main power source lines may be electrically connected by bypass power source line(s) arranged to intersect the main power source lines. Such an arrangement can minimize the difference between power supplied to emissive elements located near and far from the power source that arises from wiring resistance of the power source lines. Accordingly, the power that should be supplied can adequately be provided to each display pixel emissive element. Irregularities in the luminance between the display pixels that emit light according to supplied power can thereby be reduced, accomplishing uniform light emission within the display pixel region.
According to a still further aspect of the present invention, in any one of the above-described devices, said first and said second thin film transistors include active layers composed of poly-silicon.
Use of thin film transistors, especially those employing poly-silicon as the active layers, as elements for controlling the emissive elements can contribute to large display size and high resolution in display devices because thin film transistors are capable of high-speed operation and control emissive elements to reliably emit light during an appropriate period of time. Further, pixel driver circuits comprising poly-silicon thin film transistors created using similar processes as the TFTs within the pixel region can be integrated on the same substrate where the display pixel region is formed. This can contribute to reducing the size of margins in a display device and to reduction in manufacturing cost of the overall display device.
In another aspect of the present invention, said emissive layer may be a layer using an organic compound as an emissive material. Forming the emissive layer using an organic compound can be extremely advantageous especially in a color display device because organic compounds can provide many variations in emitted colors and a wide selection of materials.
According to another aspect of the present invention, said main power source lines and said bypass power source line are conductive line integrally formed. Alternatively, said main power source lines and said bypass power source line may be conductive lines separately formed in different processes.
In a further aspect of the present invention, said bypass power source line in the above-described device is formed in a layer located underneath said main power source lines and separated by an insulating layer, and is connected to said main power source lines via contact holes.
In a still further aspect of the present invention, said bypass power source line is formed in a same layer as a gate line.
According to a further aspect of the present invention, said bypass power source line is formed on a gate insulating film, and an interlayer insulating film that separates the active layer of said second thin film transistor and a main power source lines is provided between said bypass power source line and said main power source line as said insulating layer.
These arrangements allow efficient and reliable formation of the main power source lines and the bypass power line.